1. Field of the Invention
The present invention relates to an endoscope system for displaying information on oxygen saturation of hemoglobin in blood, a processor apparatus of an endoscope system, and an image generating method.
2. Description Related to the Prior Art
Recently, diagnoses using an endoscope have been performed commonly. In addition to normal observation of a portion of a subject using an endoscope apparatus with illumination of broadband light (white light), special observation using illumination of narrowband light has come into practice. By using the narrowband light, blood vessels in the portion being observed are highlighted in display.
Furthermore, functional information such as oxygen saturation of hemoglobin in blood and blood vessel depth is obtained from an image signal from an endoscope apparatus. The functional information is obtained based on light absorption property of blood vessels and scattering characteristic of living tissue. Then, an image representing the functional information is generated. For example, in Japanese Patent No. 2648494, colors are assigned according to different oxygen saturation levels. A pseudo-color oxygen saturation image is generated based on the colors assigned. Such oxygen saturation image facilitates finding, for example, a cancer causing a low-oxygen or hypoxic condition.
The oxygen saturation is obtained by performing image processing to an image signal. Reliability of the oxygen saturation declines when the image signal is obtained under inappropriate imaging conditions, for example, when a pixel value of the image signal is extremely high due to a large quantity of illumination light applied to the portion being observed.
To solve the problem, in Japanese Patent No. 2768936 and U.S. Pat. No. 5,956,416 (corresponding to Japanese Patent No. 3217343), a region in which a pixel value of an image signal exceeds a predetermined value is detected as an ineffective region. The ineffective region is subjected to processing (for example mask processing) to be displayed differently from an effective region in which the pixel value remains within a predetermined range. Thereby, it becomes easy for a user to distinguish a region with accurate functional information, such as the oxygen saturation, from a region with inaccurate functional information.
Because the ineffective region is clearly distinguished from the effective region in the Japanese Patent No. 2768936 and the U.S. Pat. No. 5,956,416, an appropriate threshold value for determining a boundary value between the effective and ineffective regions is important. When the threshold value is inappropriate, a region with accurate oxygen saturation may be determined as the ineffective region. Conversely, a region with inaccurate oxygen saturation may be determined as the effective region.
In addition to the intensity of the illumination light described in the Japanese Patent No. 2768936 and the U.S. Pat. No. 5,956,416, the following factors make the oxygen saturation inaccurate.    1. an artificial object captured in an image    2. dirt on mucosal surface (opaque mucosa or residue adhered to the mucosal surface)    3. different spectral distribution characteristics between spectral images in different wavelength rangesThe Japanese Patent No. 2768936 and the U.S. Pat. No. 5,956,416 cannot detect abnormality in the oxygen saturation caused by the above factors.